It is known from the prior art to apply a phosphor to an existing LED in order to convert the light spectrum emitted by the LED at least partially into another light spectrum. In this case, the radiation emitted by the LED is absorbed at least partially by the phosphor, which is therefore excited to produce photoluminescence. The resulting emission or light color of the LED module or the light source is then produced from the mixture of the transmitted (i.e. unconverted) portion of the spectrum of the LED and the emission spectrum of the phosphor.
In order to produce white light, in this case an LED which emits blue light and therefore light of a wavelength of approximately 440-480 nm is preferably used. The phosphor used is, for example, doped YAG (yttrium aluminum garnet), which, depending on the concentration, absorbs parts of the blue light emitted by the LED and converts it into predominantly yellow luminescence radiation with a high degree of efficiency. In addition, the phosphor used can be a mixture of green and red phosphor which converts the blue light of the LED at least partially into green and red luminescence radiation. Owing to the resultant additive color mixing of the colors blue, red and green, white light can therefore be produced.
In addition, it is possible to apply a three-band phosphor system to the blue LED. In this case, three different phosphors are applied to the LED which convert the emitted light of the LED into green, red and yellow luminescence radiation.
In the above-described phosphor systems, it is particularly desirable to achieve a high color rendering value (CIE Color Rendering Index (CRI)) of the LED to be produced. This is understood to be a photometric variable, which can be used to describe the quality of the color rendering of light sources of identical correlated color temperature. Depending on how many of the in total 16 standard colors are used for the calculation of the color rendering value, different color rendering values are defined, with the most common being RA8 and RA14. Spectral portions outside the visible range are irrelevant in the determination of the color rendering value.
DE 10 2006 016 548 A1 describes a phosphor which emits blue to yellow and which consists of a nitride orthosilicate and is intended to make it possible to produce color-stable, efficient LEDs with good color rendering. In addition, the phosphor is very stable to radiation, which means that it can be used in high-brightness LEDs.
In this case, a phosphor mixture with in particular three phosphors is provided in order to provide warm-white LEDs with a high color rendering index Ra of up to 97. In addition to the novel green-yellow phosphor, a red, in particular nitride phosphor is added.
DE 10 2007 001 903 A1 describes a phosphor body containing Cr(III)-activated aluminum oxide (ruby), the production thereof and the use thereof as LED conversion phosphor for white LEDs or so-called color-on-demand applications being envisaged.
The red phosphor used can be nitridosilicate. In addition, the use of a Ce-doped YAG phosphor is disclosed.
The phosphor body is preferably used for the purpose of implementing stable color loci for color-on-demand LED applications with red light portions. Accordingly, a preset color value is achieved by corresponding mixing of the ruby-containing phosphor body with a further conversion phosphor, with the deep-red emission resulting in superior color rendering.
Single-component and two-component phosphor systems have the disadvantage in comparison with three-component phosphor systems that, at a constant color temperature, at a constant wavelength and at a constant power of the LED chips and with an absolutely identical LED package, apart from random fluctuations, it is only possible to achieve one color rendering value and one luminous flux. Precise setting of the color rendering value is therefore not possible with single-component and two-component phosphor systems.
The three-band phosphor systems known from the prior art have the disadvantage that the luminous efficacy is reduced at the same time as the color rendering value of the LED increases. In order to achieve a luminous efficacy which is as high as possible for an LED module, the color rendering value should therefore not be above predefined requirements in order to avoid the disadvantage associated with the luminous efficacy. Precise setting of the color rendering value is therefore desirable.
One problem in particular with the multiband phosphor systems is also the fact that the light conversion by the phosphors depends on the temperature in the color conversion layer in which the phosphors are located. In addition, the current flowing through the LED has a decisive influence on the light conversion within the color conversion layer. In the known multiband phosphor systems, a certain decrease in the performance of the color conversion at relatively high temperatures and/or relatively high currents through the LED tends to take place (quenching or saturation effects). As a result of this decrease in the phosphor efficiency, the color locus of the emitted light of the LED module shifts, this color locus being expressed by x and y coordinates in the CIE chromaticity diagram.
The invention therefore provides a three-band phosphor system which is stable with respect to the temperature and the current flow through the LED for an LED module, which three-band phosphor system enables a luminous efficacy which is as high as possible given a predefined color rendering value of the LED module.